1. Field of the Invention
The invention relates to a projection objective for microlithographic projection exposure apparatuses, such as those used for the production of large-scale integrated electrical circuits and other microstructured components.
2. Description of Related Art
Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers to a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV) spectral range.
The wafer coated in this way is subsequently exposed in a projection exposure apparatus. A pattern of diffracting structures, which is arranged on a mask, is projected onto the photoresist with the aid of a projection objective. Since the imaging scale is generally less than 1, such projection objectives are also often referred to as reduction objectives.
After the photoresist has been developed, the wafer is subjected to an etching process so that the layer becomes structured according to the pattern on the mask. The remaining photoresist is then removed from the other parts of the layer. This process is repeated until all the layers have been applied to the wafer.
One of the essential aims in the development of projection exposure apparatuses used for production is to be able to lithographically define structures with smaller and smaller dimensions on the wafer. Small structures lead to high integration densities, and this generally has a favourable effect on the performance of the microstructured components produced with the aid of such systems.
The size of the structures which can be defined depends primarily on the resolution of the projection objective being used. Since the resolution of the projection objectives is proportional to the wavelength of the projection light, one way of increasing the resolution is to use projection light with shorter and shorter wavelengths. The shortest wavelengths used at present are in the deep ultraviolet (DUV) spectral range, namely 193 nm and 157 nm.
Another way of increasing the resolution is based on the idea of introducing an immersion liquid with a high refractive index into an intermediate space which remains between a last lens on the image side of the projection objective and the photoresist or other photosensitive layer to be exposed. Projection objectives which are designed for immersed operation, and which are therefore also referred to as immersion objectives, can achieve numerical apertures of more than 1, for example 1.3 or 1.4. The immersion moreover not only allows high numerical apertures and therefore an improved resolution, but also has a favourable effect on the depth of focus. The greater the depth of focus is, the less stringent are the requirements for exact positioning of the wafer in the image plane of the projection objective.
Carrying out immersed operation, however, requires considerable extra outlay on construction and process technology. For example, it is necessary to ensure that the optical properties of the immersion liquid are spatially homogeneous and constant as a function of time, at least in the volume exposed to the projection light, even if the substrate with the photosensitive layer applied to it moves relative to the projection objective. The technological difficulties associated with this have not yet been resolved satisfactorily.
It has therefore been considered expedient that projection objectives designed for dry operation, which will be referred to below as “dry objectives” for short, should be operated in immersion only during particularly critical process steps. Of course, with a objective designed for dry operation it is not possible to increase the numerical aperture since this requires a different configuration of the projection objective. Nevertheless, a higher depth of focus is achieved even in the immersed operation of dry objectives, and this can be advantageous in particularly critical process steps. The dry objective may be used without an immersion liquid in the less critical process steps, so that the exposure of the wafer is simplified considerably and, as a general rule, can be carried out more rapidly.
However, the introduction of an immersion liquid into the immersion space will affect the imaging by the projection objective in such a way that major adjustments to the dry objective have to be carried out before the immersed operation commences. Such adjustments are described in US 2004/109237 A1. The original state has to be restored for a subsequent change to dry operation, which again entails significant costs. Of course, it is also possible to configure the dry objective a priori so that it can be operated in immersion. The numerical aperture must then remain less than 1, since otherwise total reflection would occur at particular optical surfaces during the dry operation. But in this case, too, adjustments are necessary for a change to dry operation since the removal of immersion liquid naturally also affects the imaging.